A standard extruding press has a housing defining an axis, an end frame displaceable axially on the housing, and a plurality of cylinders on the housing each having a piston having a piston rod connected to the end frame. Each piston subdivides the respective cylinder into front and rear compartments. A tool is fixed on the housing and a workpiece can be positioned between the tool and the end frame which can carry another tool. When the front compartments are pressurized the workpiece is pressed rearward against the tool can be extruded through it. The use of plural cylinders (as described in Hydraulische Pressen und druckflussigkeitsanlagen by Ernst Muller, volume 3, pages 83 and 84 and German patent 510,895) eliminates the need for a separate guide frame on the press.
Separate respective fluid-powered actuators are braced between the housing and the pistons so that when pressurized they displace the end frame forward away from the tool at a considerably higher speed than the speed of the end frame when it is forcing the workpiece through the frame-mounted tool. To make this possible there are passages extending between the compartments and controllable valves in the passages for permitting free fluid flow between the compartments during this fast forward movement.
Thus it is standard to provide auxiliary fluid actuators that can rapidly open the press and bring it to the initial closed position at the start of a pressing cycle. The problem with this is that substantial volumes of hydraulic fluid must be moved into and out of the two compartments of the heavy-duty double-acting ram that is used for this type of machine. As a result of the high-volume flow, there is turbulence and air bubbles can be entrained or sucked into the fluid, compromising operation of the machine.
Accordingly EP 0,822,017 proposes a system where the actuator has a piston with a pair of oppositely directed piston rods projecting axially out of opposite ends of the cylinders. Shunt conduits are provided between opposite ends of the cylinders with valves that can be opened so when the pistons are quick-stroked to their open positions fluid can flow through the shunt between the compartments. The function of the front piston rod is to reduce the effective surface area of the rear piston face, that is the area exposed axially in the rear compartment, so that is it generally equal to that of the front piston face, so that although there is some flow between compartments, the total volume of the compartments does not change significantly as the piston moves.
The problem with such a system is that there are several potential leak points that serve little function. Where the rear piston rod projects through the rear end of the cylinder must be meticulously sealed. This problem is complicated by the fact that the piston rods are exerting enormous forces and are subject to some bending. As a result the rear piston rod can jam in the rear cylinder wall or at least subject one side of the seal to considerable wear if the piston gets slightly canted.